1. Field of The Invention
The subject invention relates to a controller for supplying a switching signal to a switched mode power supply.
2. Description of The Related Art
Switched mode power supplies are extensively used in applications where efficient and compact power conversion is required. While there are many topologies and implementations of switched mode power supplies, all variations have square-wave signals for driving the power switches. These square-wave signals are normally generated by a pulse-width modulation controller and amplified by a buffer/driver which interfaces with the gate/base of the power switches. Low complexity, ease of implementation, well-understood operation and commercial availability of components have made square-wave control the de-facto standard in switched mode power supply applications.
FIG. 1 shows a block diagram of a flyback switched mode power supply widely used in television receiver power supply applications and indicates the square-wave control and drive as functional blocks.
It has been identified that the square-wave control is a major contributor of radiated EMI noise in the television receiver power supply. This can be explained in terms of sharp di/dt and dv/dt transitions caused by the square-wave control, The parasitic elements in the drive circuit are excited during transitions and create high frequency ringings which compound the EMI problem. The net effect is that the contribution of the control circuit to the radiated EMI signals is significant. This can result in noise visible on the display screen of the television receiver during reception of low level signals.
In order to minimize this noise, most other approaches use power stage elements (such as snubber, resonant techniques, etc.) to try and reduce the noise. However, they do not have any impact on the drive circuit transitions which contribute significantly to the noise generation. The only other known possible approach to reduce the noise contribution is to introduce RDC damping circuits (also know as gate slow-down approaches) in the drive path. While these circuits have low complexity, they have limited effectiveness due to two reasons. First, they still do not eliminate fast transitions (sharp edges) in the control circuit. Second, they add significant dissipation in the power device when the damping level is increased.